Traditional communication networks such as, for example, telephone or cable television are by nature intended each for a particular use and thus individually ill-adapted to executing the tasks which the other networks execute. Networks of a novel type have thus appeared and notably those operating in an Asynchronous Transfer Mode (ATM). According to this ATM, the transmission of a signal is effected in a digital data stream which is subdivided into fixed-size cells (53 octets). Each cell itself is formed by a header (5 octets) which contains the necessary information for the transfer of the cell, and a payload (48 octets), containing the useful information generated by a source. All the generated cells are then multiplexed via queues which are joined together to form an ATM network.
In the same ATM network, very different types of data may be present having specific constraints for each type. For example, the transmission of voice is to be effected in real time, but transmission errors are permitted within certain limits. Conversely, no real-time constraint affects a file transfer, but corrections of transmission errors are indispensable. When the transmitted data are video data according to the MPEG2 standard, a transmission in real time and having a smallest possible error rate is necessary.
For permitting data of such diverse nature to coexist in the network, a featuring of the traffic is basically ensured, by carrying out two operations called Connection Admission Control (CAC) and User Parameter Control (UPC). The operation of admission control, which the invention does not relate to, consists of verifying, when a user A wishes to communicate with a user B, whether the network has enough resources for effectively authorizing the connection of A and the corresponding data transfer to B, taking account of the connections already established across the network at that particular instant and of the necessity of not producing disturbances for these established connections.
For the network that establishes that a connection has been admitted between two users A and B, the operation of user parameter control (UPC) to which the present invention relates, consists of verifying that the sending user well respects the transmission contract which has been the object of the preceding admission control. Indeed, if this contract is violated either by accident (malfunctioning of the software) or voluntarily (fraudulent action), the additional resources which the network is to assign to these nonauthorized data transfers are taken from those that had already been assigned to the connections admitted by contract and respecting their transmission contract. Furthermore, an unforeseen data congestion may result from this violation of contract and corrupt wholly or in part the operation of the network. Indeed, the queues are always of limited length, and, finally, more and more data cells occur which are to be discarded.
A very frequently used method of implementing this user parameter control operation is described in patent application GB-2287854. Better known under its English name of leaky bucket algorithm, this method provides an authorization of access of a message, for example, an ATM cell, to a network only as a function of the residual level of a token reserve previously assigned as a function of a certain number of criteria notably linked with the featuring of the traffic. Initially, a time period T(UPC) is defined and a reserve of N tokens is formed for the duration of this period. A message, generated by a data source can then have access to a network only utilizing one of these tokens and it is thus a maximum of N messages that will be able to penetrate into the network during this period T(UPC). If the checking of the tokens, realized by a counter, shows that no token is available any longer for a new requested access, the message is rejected, or at any rate labeled to be transmitted only with a lower priority and on condition that it does not cause any problem for the operation of the network, even if the message is admitted.
This control method, however, has the following drawback. As only a maximum of N tokens can be disposed of at an arbitrary instant, no new token is created if no token has been used during the period T(UPC) that is completed (if certain tokens have been used, all the non-used tokens are lost). Therefore, in the situation where no token whatsoever has still been used and where suddenly a burst of N messages occurs, each of them uses a token and all messages access the network, but the token reserve is emptied and any new access is, however, forbidden for a rather long period of time (certain networks furthermore provide that the transmission contracts are not concluded when the messages thus occur as bursts, very irregularly running the risk of completely disturbing the network).